A group of Swedish researchers began to wonder recently whether alterations in the activity of genes and proteins inside our muscles might help to explain whether and why our muscles remember how to be strong.

To find out, they began by recruiting 19 young men and women who had never played sports or formally exercised at all, so that their muscles were new to formal weight training. They checked these volunteers' current muscular strength and size, and then had them start training a single leg.

To accomplish this, the young men and women completed increasingly strenuous leg presses and leg extensions using only their right or left leg, while the other limb lollygagged to the side. These one-legged workouts continued for 10 weeks, at which point the researchers re-measured muscles, and then the volunteers stopped their training completely for 20 weeks.

After this layoff from working out, they returned to the lab, where the scientists checked the current state of their leg muscles, took muscle biopsies from both legs and had them complete a strenuous leg workout, using both legs this time. Afterward, the researchers biopsied the muscles again. Then they checked the levels of a wide array of gene markers and biochemical signals within the volunteers' muscle cells that are believed to be related to muscle health and growth.

They found telling differences between the legs that had trained and those that had not, both before and after the lone training session. For one thing, the previously trained leg remained sturdier, having retained about 50 percent of its strength gains during the 20 weeks without exercise.

The molecular differences from leg to leg were more complex, with some genes showing greater activity in the trained leg and others less, and some biochemicals being more abundant there and others more uncommon, compared to the untrained limb. Some of these variances appeared in each leg before the workout, indicating that the trained muscles had become and remained subtly distinct, even after 20 weeks without exercise. Other molecular changes cropped up after the workout, with each leg responding a bit differently to the exertion.

Taken as a whole, the scientists concluded, the trained leg's genetic activity suggests that its muscle cells had become genetically and metabolically more ready to strengthen and grow than the cells in the leg that had not trained before.– These findings "support the idea that muscle memory could occur at the gene and protein level," says Marcus Moberg, an assistant professor at the Swedish School of Sport and Health Sciences in Stockholm, who led the new study.

The scientists did not track the volunteers through subsequent workouts for this study, though, and cannot say how quickly strength and mass might bounce back. They also studied weight training, not endurance exercise, and only in young people without health problems.– But the results remain encouraging, especially for everyone confined at home now. When gyms reopen, our muscles should rapidly rebound, Dr. Moberg says.